The demand for higher reactor throughput continues to increase for a variety of industrial processes linked to oil and gas recovery, alternative fuel production, sustainability of the environment and process emissions. Such demands are partially driven by the ever-increasing cost of fuel and the need for various chemical feed stocks. One example is the demand for larger cryogenic air separation units (ASUs) to meet the growing needs for large quantities of oxygen and nitrogen used in various industrial process industries. ASUs require front end purification reactors (adsorption vessels) to purify the feed air stream by removing carbon dioxide, water, trace hydrocarbons and other contaminants prior to entering the ASU. Larger ASUs require larger “prepurification units”, as they are commonly known to treat the incoming feed air prior to cryogenic distillation. This presents a challenge to reactor designers when trying to control the size of the reactor since higher throughput of feed air demands a proportional increase in the frontal flow area provided by the vessels resulting in larger, more costly vessels.
Gas purification, separation or reaction processes using active materials such as adsorbents and/or catalysts are well known in the art and there are several reactor vessel designs in use today for these types of processes. Examples include both vertically and horizontally oriented cylindrical vessels with upward air flow through the bed of adsorbent material, reactant and/or catalytic material during purification, separation or chemical reaction. A third type of vessel, as employed herein, is oriented with a vertical central or longitudinal axis and an internal design that directs the process gas flow radially through the bed. This radial flow design consists of a pressure vessel enclosing gas permeable concentric inner and outer baskets to contain a bed of one or more layers of active material. Radial flow designs offer the ability to increase frontal flow area by increasing the height of the vessel without substantially altering the vessel footprint (ground area requirements). Furthermore, a radial flow design offers a more efficient means of increasing flow area for a compared to either horizontal or axial flow reactor designs.
Radial flow reactors typically operate continuously or in cyclic mode, depending upon the gas treatment process. Many processes, such as adsorption processes, operate cyclically in either pressure swing (PSA), vacuum swing (VSA), temperature swing (TSA) mode or in combinations of these modes wherein one or more components of the feed stream are adsorbed during the adsorption step and then desorbed or otherwise flushed from the adsorbent during the adsorbent regeneration step. When thermal variations accompany these cyclical processes, such as in TSA processes, the changes in temperature cause bed and vessel components in contact with such thermal variations to expand and contract. Depending upon the configuration of the internal components, as well as their manner of connection to the vessel, these thermal expansion and contraction induce loads within the bed are subsequently transferred to the internal components of the reactor. Such thermally induced loads create significant mechanical stresses on all elements of the internal basket assemblies, the magnitude of such induced loads increases with increasing temperature difference. Axial and radial displacement of the basket walls may also result in compression of the bed of active material and the material particles may migrate or be damaged as a result of the basket wall movement. In the worst case, these effects can cause physical breakdown of the active material and/or mechanical failure of the basket assemblies.
By way of example, the particular problems associated with radial flow reactors are more fully described in a typical thermal swing air purification process. It is advantageous to operate such a reactor by introducing the feed air into the outermost passage between the vessel shell and the outer basket during the adsorption step and by introducing the regeneration gas into the passage enclosed by the inner basket during the desorption step. Thus, feed air is purified by passing radially through the adsorbent bed toward the central axis of the reactor. Regeneration gas passes radially through the bed in the opposite direction to desorb the contaminants and renew the bed for the subsequent cycle. Adsorption of the contaminants from the feed gas occurs at substantially ambient temperature. Regeneration is performed using a thermal pulse wherein heated gas is first introduced for a specified time followed by cold gas, where the cold gas is at about the same temperature as the feed gas. During the heating phase of regeneration, a heat front develops at the inner basket wall and then travels outwardly and radially through the bed. The part of the bed ahead of the heat front remains near ambient temperature, while the part of the bed already traversed by the heat front is at the hot regeneration temperature. When this heat front reaches an intermediate radial position within the bed, the cold gas is introduced to the inner basket space. This gas is warmed as a cold front develops at the rear of the heated zone. The resultant thermal pulse then continues to push the heat front through the remaining adsorbent as the stored energy is consumed by desorbing the remaining contaminants in the bed. The vessel shell and heads remain predominantly at ambient temperature during the entire operation of a cycle, i.e. the ends and shell of the vessel have little contact with the hot gas, remaining at a relatively constant temperature over each cycle, and therefore remain fixed in space. Conversely, the internal components of the reactor experience these temperature variations directly, resulting in thermal expansions and contractions and the associated induced loads and stresses.
The reactor and its internal components must therefore be designed to minimize and accommodate radial and axial movement so that the mechanical integrity of the basket assemblies and the active material contained within the baskets is maintained throughout the thermally induced loads and stresses. Further, the thermally induced mechanical stresses limit the temperature range over which conventional radial flow reactors may operate and these limitations are amplified as the size of the reactor increases thereby limiting the size and application of the reactors.
Thus, there is significant motivation to improve the mechanical design of radial flow reactors to affect greater operational reliability, lower cost and increased process flexibility while still limiting the overall footprint of the reactor vessel. Further, the present reactor is designed to permit a simple and effective means for addressing the problems associated with thermally induced mechanical stresses and thereby enable the aforementioned improvements.
The teachings in the art are varied and inconsistent with respect to the design of radial flow reactors; particularly for vessels undergoing thermal cycling. Conventional cylindrical reactor designs typically include an internal assembly of at least two concentric porous wall baskets with the active material contained in the annular space formed between these baskets. The baskets and vessel shell generally share the same longitudinal axis. Beyond these commonalities, the teachings diverge significantly in describing a variety of means to support the basket assembly. For example, the baskets are either suspended from only the top end of the vessel, supported at only the bottom end, or fixed between both ends of the vessel.
U.S. Pat. No. 4,541,851 discloses a vessel having two concentric layers of adsorbent, each layer contained between two concentric cylindrical grates. Three cylindrical grates are concentric about the same longitudinal axis as the vessel enclosing them. The intermediate grate is axially rigid and radially flexible while the inner and outer grates are axially flexible and radially rigid. All three grates are interconnected rigidly to the vessel shell at their upper end and interconnected rigidly to a solid floating bottom plate at their lower end. The assembly of the three concentric grates is thus suspended inside the vessel from the top head so that the weight of the grates, bottom plate and the adsorbent material is primarily carried by the axially rigid intermediate grate. The intermediate grate expands and contracts in the axial direction. The axial movement of the flexible inner and outer grates follows that of the intermediate grate. The inner and outer grates expand and contract in the radial direction and alternately squeeze and release the adsorbent bed in the radial direction upon heating and cooling. The intermediate grate expands/contracts radially within the bed since it is flexible in the radial direction, and, as a result, imparts very little additional radial squeezing force on the adsorbent bed.
U.S. Pat. No. 4,541,851 discloses in a second embodiment a vessel having three concentric layers of adsorbent and four permeable grates. The inner and outer grates are rigid in both the axial and radial direction and the two intermediate grates are rigid in the axial direction and flexible in the radial direction. All four grates are interconnected rigidly to the shell at their lower ends. At their upper ends, all four grates are free to move in the axial direction with the three outer grates able to slide axially in guides, while the innermost grate terminates in a dome that is able to move freely in the axial direction. Two or more layers of adsorbent can be used in this configuration. As thermal pulses move through the adsorbent bed, the grates alternately are heated and cooled. The design allows each of the grates to expand freely and independently of each other in the axial direction. The radial squeezing forces are transmitted to all three layers of adsorbent because of the circumferential flexibility of the two intermediate grates. Additional details are associated with this design are described by Grenier, M., J-Y Lehman, P. Petit, “Adsorption Purification for Air Separation Units,” in Cryogenic Processes and Equipment, ed. by P. J. Kerney, et al. ASME, New York (1984).
U.S. Pat. No. 5,827,485 discloses a vessel containing an annular adsorption bed which is bounded by inner and outer baskets. A single layer of adsorbent is taught which is contained between the two permeable concentric baskets, both of which are flexible in the axial direction and rigid in the radial direction. At least one of the baskets is rigidly fastened to the top end of the vessel. The inner basket is rigidly connected at its lower end to a bottom support member and further supported on lower a hemispherical cap of the shell by ribs arranged like a star. The outer basket is directly supported at its lower end by the bottom cap. A ratio of coefficients of thermal expansion of the baskets relative to that of the “free flowing” active material or adsorbent is claimed to be in the range of 0.25-2.0. It teaches that this combination of features essentially eliminates the relative motion of particles of the free flowing material due to the thermal cycling of the baskets. It also suggested that pre-stressing at least one of the baskets reduces the axial stresses that develop within the baskets as a result of thermal cycling, although no description of the method to pre-stress is provided. Additional details are also described by U. von Gemmingen, “Designs of Adsorptive Dryers in Air Separation Plants”, Reports on Science & Technology, 54:8-12 (1994).
U.S. Pat. No. 6,086,659 discloses a radial flow adsorption vessel that has a plurality of grates, wherein at least one of the grates is flexible in both the axial and radial directions. This “bidirectional flexibility” is preferably imparted to at least one of the intermediate grates. Many combinations of axial/radial flexibility/rigidity are offered for the inner and outer baskets. The grates are rigidly attached to both the top of the vessel and to a bottom plate. The bottom plate may be floating or semi-rigidly or rigidly attached to the bottom head of the vessel. One or more intermediate grates are disclosed as a means to contain various layers of adsorbents within the vessel.
German Patent No. DE-39-39-517-A1 discloses a radial flow vessel having a single layer of adsorbent contained between two concentric permeable grates, both of which appear to be rigid in both the axial and the radial direction. The outer basket is rigidly connected to the top end of the vessel and to a floating bottom plate. The inner basket is flexibly connected to the top end of the vessel through the use of an expansion bellows or a sliding guide. The lower end of the inner basket is connected rigidly to the floating bottom plate. The entire basket assembly is thus suspended from the top end of the vessel with the outer basket carrying the weight of the assembly and the adsorbent contained therein. The inner grate is enclosed on the adsorbent side with a gas permeable compressible material or mat to absorb any radial compressible forces resulting from thermal expansion and contraction.
As illustrated above, the patent art teaches many variations within basic design configurations wherein inner, outer, and/or intermediate baskets may possess axial flexibility, radial flexibility, or combinations thereof. All of these designs have various deficiencies, most notably a continuing problem with thermally induced stress, shearing, and possible damage to the equipment and active material. Notwithstanding these teachings, there is no clear direction for the design of a radial flow reactor to mitigate or eliminate these problems. Moreover, there are no teachings on methods for pre-stressing the internal baskets or on reactors designed for this purpose.
The present radial flow reactor is designed such that the internal basket assembly containing the bed of active material is rigidly supported at both the top and bottom ends of the vessel. The size and geometry of the perforations in the basket walls largely dictate the amount of axial flexibility and radial rigidity that results to minimize thermally induced movement and to control stresses and loads, thereby mitigating axial and radial buckling of these walls. The present reactor also provides a simple and advantageous means of pre-stressing the baskets which is used herein to describe the act of placing the baskets in tension at ambient temperature.